This invention relates generally to an alignment and exposure apparatus for use in the manufacture of semiconductor devices, and more particularly to focus adjustment in an alignment and exposure apparatus of the projection exposure type.
In order to meet the recent demands for further miniaturization and higher integration of circuit patterns of large scaled integrated devices (LSI) or very large scaled integrated devices (VLSI), there has already been proposed an alignment and exposure apparatus of the projection type wherein a projection optical system is employed to project the image of a circuit pattern formed on a photomask or reticle onto a wafer for exposure thereof. This type of alignment and exposure apparatus can be generally classified into two types: one is a reduction projection system wherein the image of a reticle having one or more master patterns is projected onto a part of a wafer through a lens system with a predetermined demagnification (reduced scale), and by a step-and-repeat process, the entire surface of the wafer is successively exposed; and the other is a mirror projection system wherein a fixed mirror imaging optical system is employed to project the image of a region of a photomask held by a photomask holder onto a wafer held by a wafer holder, and, while effecting the exposure, the photomask holder and the wafer holder are moved as a unit relative to the mirror imaging optical system so that the image of the entire region of the photomask is projected onto the entire surface of the wafer for exposure thereof.
One of the important features to be possessed by these alignment and exposure apparatuses is a resolving power required for accurate reproduction of the image of the photomask or reticle (which will hereinafter be referred to simply as a "mask") on the wafer surface. The term "resolving power" means the resolution characteristics of the apparatus which is an index of how fine a pattern can be reproduced during exposure of the wafer to the mask pattern.
This resolving power of the projection type alignment and exposure apparatus is determined by various factors. Major ones of these factors are fundamental characteristics of the projection optical system such as an effective F-number (Fe) and a numerical aperture (NA), and accuracy and assembling precision of various optical elements such as lenses and mirrors. Moreover, the degree of stabilization of the focal distance including the optical system between the mask and wafer is also one of the major factors. Since, in general, the depth of focus of the projection optical system is of the order of several micron millimeters, focusing must be precisely achieved. In addition thereto, it is also very important that the focusing is achieved over the entire wafer surface, since there may be unevenness in the thickness or surface flatness of the wafer.
Usually, the focusing or focus adjustment has been achieved in this type of alignment and exposure apparatus by, first, correcting any warp of the wafer from behind with the use of a super-flat plate (or a wafer chuck) having a flatness-correction function, and by, second, moving the wafer so that the upper surface thereof is brought into abutment with three projections formed on a reference surface of a wafer disk which is located at a determined distance from the mask, whereby the wafer is positioned in its place. This technique has however some disadvantages. That is, when the wafer abuts against the projections, an adhesive photoresist applied to the wafer surface will be adhered to the protrusions formed on the reference surface. The amount of resist adhesion will increase with the increase in number of the wafers processed to make the distance between the mask and wafer unstable, which results in the possibility of defocus. Further, the areas on the wafer which are in contact with the three projections formed on the reference surface are not exposed to the mask pattern, which results in a poor yield of the semiconductor devices.
There has also been proposed a non-contact focusing technique such as disclosed in Japanese Laid-Open patent application No. 1229/1982 or U.S. Pat. No. 4,344,160. According to this technique, the wafer is kept out of contact with the reference surface, and, alternatively, a displacement gauge is employed to measure the distance between the reference surface and the wafer so that the wafer is moved toward or away from the reference surface by a distance corresponding to the difference between the measured value and a predetermined value. In this technique, however, a mechanism for displacing the wafer must be incorporated into the wafer chuck, so that the structure of the wafer chuck is complicated.
Moreover, if the alignment and exposure apparatus is arranged so that a plurality of wafer chucks are provided on a moving mechanism, such as a turntable, so that, during exposure of one wafer, loading, unloading, positioning or the like of the other wafers are performed in order to increase the throughput, the alignment and exposure apparatus must include a plurality of focusing mechanisms for the respective wafer chucks. This leads to a substantial increase in the manufacturing cost.